Alloys strengthened by nitride particles (referred to as “NDS”, standing for “Nitride Dispersion Strengthened”), have improved mechanical properties compared with master alloys, among others better mechanical tensile, creep, compressive or fatigue strength.
These properties may be further improved by reducing the size of the dispersed particles.
Numerous studies thus aim to develop a production method of an NDS alloy with particles of reduced size.
Among these methods, gas nitriding is frequently employed. The document “Johansson et at., Nitrogen alloyed stainless steel produced by nitridation of powder, Metal Powder Report, 1991, 46 (5), pp. 65-68”, describes a method in which an austenitic steel powder containing titanium is heated to around 1000° C. under a pure dinitrogen (N2) atmosphere in order to form precipitates of an intermediate nitride, chromium nitride Cr2N. Under the action of a supplementary heat treatment at 1200° C., these precipitates are then dissolved in order to result in an alloy strengthened by titanium nitride dispersions.
The supplementary heat treatment of this nitriding method nevertheless has the drawback of producing dispersions of an average size that may be as large as 300 nm. This large size of the dispersion has a tendency to degrade the mechanical properties of the strengthened alloy.
Another type of production method used for an NDS alloy involves powder metallurgy. In the document U.S. Pat. No. 4,708,742, a powder of a nitrogen donor compound (such as Cr2N) is co-milled with a powder intended to form the metal matrix of a strengthened alloy. The blend of powders obtained is subjected to heat treatment in order to decompose the nitrogen donor so that the dinitrogen thus available forms a nitride with one of the elements of the metal matrix. After consolidation of the blend of powders, an alloy strengthened by nitride dispersions is obtained.
The heat treatment intended to produce dinitrogen by decomposition of the nitrogen donor means that this powder metallurgy method may be assimilated to a nitriding method.
The requirement to have available an intermediate nitride such as Cr2N before forming the final metal nitride therefore also has an unfavorable effect on the size of the dispersed nanoparticles, which is at best around one micrometer.
The aforementioned methods of the prior art therefore have a particular drawback in that they do not make it possible to produce a strengthened alloy in which the nanoparticles mainly have a reduced average size, typically less than 50 nm.
In addition, the requirement to proceed by an intermediate nitride means that these methods are subject to parasitic reactions that make it difficult to control the composition and quantity of the particles that are present in the strengthened alloy obtained.